Pacem in terris (lit.'Peace on Earth') was a papal encyclical issued by Pope John XXIII on 11 April 1963 on the rights and obligations of individuals and of the state, as well as the proper relations between states. It emphasized human dignity and equality among all people, and made mention of issues such as the rights of women, nuclear non-proliferation, and the United Nations, all of which it endorsed. It was the last encyclical drafted by John XXIII, who had been diagnosed with cancer in September 1962 and died two months after the encyclical's completion. Biographer Peter Hebblethwaite called it Pope John's "last will and testament". Published on Holy Thursday, the Pope called it his "Easter gift".

Due to its importance and popularity, Pacem in Terris is deposited at the UN archives.

Pope John XXIII (Latin: Ioannes XXIII; Italian: Giovanni XXIII; born Giuseppe Angelo Roncalli, Italian: [ˈandʒelo dʒuˈzɛppe roŋˈkalli]; 25 November 1881 – 3 June 1963) was head of the Catholic Church and sovereign of the Vatican City State from 28 October 1958 until his death in 1963. Giuseppe Angelo Roncalli was one of thirteen children born to a family of sharecroppers who lived in a village in Lombardy. He was ordained to the priesthood on 10 August 1904 and served in a number of posts, as nuncio in France and a delegate to Bulgaria, Greece and Turkey. In a consistory on 12 January 1953 Pope Pius XII made Roncalli a cardinal as the Cardinal-Priest of Santa Prisca in addition to naming him as the Patriarch of Venice. Roncalli was unexpectedly elected pope on 28 October 1958 at age 76 after 11 ballots. Pope John XXIII surprised those who expected him to be a caretaker pope by calling the historic Second Vatican Council (1962–1965), the first session opening on 11 October 1962.

John XXIII made many passionate speeches during his pontificate. His views on equality were summed up in his statement, "We were all made in God's image, and thus, we are all Godly alike." He made a major impact on the Catholic Church, opening it up to dramatic unexpected changes promulgated at the Vatican Council and by his own dealings with other churches and nations. In Italian politics, he prohibited bishops from interfering with local elections, and he helped the Christian Democratic Party to cooperate with the socialists. In international affairs, his "Ostpolitik" engaged in dialogue with the communist countries of Eastern Europe. He especially reached out to the Eastern Orthodox churches. His overall goal was to modernize the Church by emphasizing its pastoral role, and its necessary involvement with affairs of state. He dropped the traditional rule of 70 cardinals, increasing the size to 85. He used the opportunity to name the first cardinals from Africa, Japan, and the Philippines. He promoted ecumenical movements in cooperation with other Christian faiths. In doctrinal matters, he was a traditionalist, but he ended the practice of automatically formulating social and political policies on the basis of old theological propositions.He did not live to see the Vatican Council to completion. His cause for canonization was opened on 18 November 1965 by his successor, Pope Paul VI, who declared him a Servant of God. On 5 July 2013, Pope Francis – bypassing the traditionally required second miracle – declared John XXIII a saint, based on his virtuous, model lifestyle, and because of the good which had come from his opening of the Second Vatican Council. He was canonised alongside Pope John Paul II on 27 April 2014. John XXIII today is affectionately known as the Good Pope (Italian: il Papa buono).